This invention relates to pneumatic servo boosters and, particularly to pneumatic servo boosters of the kind including a shell housing consisting of front and rear shells, a valve body slidably extending through an opening formed in the rear shell, a flexible diaphragm connected to the valve body and to the inner circumference of the shell housing and partitioning the interior of the shell housing into two chambers, a valve mechanism incorporated in the valve body and controlling the pressure difference between the two chambers, and at least one reinforcing rod extending between the front and rear shells and through the interior of the shell housing.
The valve mechanism conventionally comprises a poppet valve, a valve seat formed on the valve body, and another valve seat formed on a plunger which is connected to an input rod and is slidably mounted in the valve body. When the poppet valve is spaced from the valve seat of the valve body and is seated on the valve seat of the plunger, the two chambers in the shell housing are communicated with each other and are maintained at a first referential pressure such as a vacuum pressure, and when the poppet valve engages with the valve seat of the valve body and separates from the valve seat of the plunger, the communication between the two chambers is intercepted and a second referential pressure such as atmospheric pressure is introduced into one chamber, such as the rear chamber, while the other chamber is permanently maintained at the first referential pressure.
In the non-actuated condition of a conventional servo booster of the aforementioned kind, the poppet valve is spaced from the valve seat of the valve body by a predetermined small clearance, and seats on the valve seat of the plunger, whereby the first referential pressure prevails in both chambers. The clearance is defined as the maximum clearance between the poppet valve and the valve seat of the valve body in the return stroke of the servo booster, and thus, it is preferable to increase the clearance for improving the responsiveness of the servo booster in the return stroke. However, in actuating the servo booster it is necessary to firstly take up the clearance, i.e., the clearance constitutes an ineffective stroke in the actuating stroke of the servo booster, thereby deteriorating the responsiveness and the pedal feeling of the servo booster in the actuating stroke.
The reinforcing rod is effective to increase the mechanical strength of the shell housing, and is particularly advantageous when the rear end of the servo booster is secured to a body of a vehicle and a master cylinder of a hydraulic braking system is secured to the front end of the shell housing and is supported thereon in a canti-lever style.